Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as a metal, and propelling the softened material in particulate form against a properly prepared surface which is to be coated. The heated particles strike the surface where they quench and bond thereto. In one type of thermal spray gun, a powder of the coating material is fed axially through a low velocity combustion flame. A plasma spray gun utilizes a high intensity arc to heat inert gas in the gun so as to effect a high velocity gas stream ("plasma") into which powder is injected. In a wire type of thermal spray gun, a wire is fed axially through an oxygen-acetylene (or other fuel gas) flame which melts the wire tip. An annular flow of compressed air "atomizes" the molten wire tip into small droplets or softened particles, generally between one and 150 microns in size. Another type is an arc gun in which two wires converge to where an arc between the wires melts the tips, the molten material again being atomized and propelled by compressed air.
High velocity oxygen-fuel ("HVOF") powder thermal spray guns have recently become practical, examples being described in U.S. Pat. Nos. 4,416,421 and 4,865,252. Combustion is effected at high pressure within the gun. With a feed of powder or wire, the combustion effluent is directed through an open channel to produce a high velocity spray steam that results in particularly dense coatings. In most cases gas fuel is used, but liquid fuel is an alternative as suggested in the '421 patent.
German patent No. DE 38 42 263 C1 discloses HVOF spraying of molybdenum with molybdenum oxide. U.S. Pat. No. 5,006,321 discloses a method of producing glass mold plungers with self-fluxing alloy and carbide using HVOF. U.S. Pat. No. 5,080,056 teaches the spraying of cylinder bores and piston skirts of internal combustion engines with aluminum bronze using an arc wire gun or an HVOF type of gun.
Special steps must be taken to assure bonding of spray material to metal substrates. A common method of surface preparation is to roughen the surface with grit blasting. Such blasting is an added step which increases coating costs. In some cases, for example in engine cylinder bores, there is danger of grit particles remaining imbedded to later cause scoring or even destruction. Therefore it is desirable to eliminate the blasting step.
Some thermal spray materials bond well to a smooth, clean substrate, for example sprayed molybdenum wire as disclosed in U.S. Pat. No. 2,588,422 for producing a bond coat which is overcoated with a non-bonding type of thermal spray coating of choice. Molybdenum coatings also proved to provide low scuff wear resistance, and have been in common use on piston rings for internal combustion engines.
U.S. Pat. No. 3,322,515 discloses composite powders of aluminum and another selected metal such as nickel or chromium, to effect an intermetallic compound with an exothermic reaction during flame spraying by a wire, plasma or powder combustion gun. The results generally are improved bonding to smooth machined surfaces. It is stated in the patent (column 4, lines 5-17) that iron is not a satisfactory component for such a composite material, although iron may be combined with another metal sufficient to provide an effective exothermic reaction.
U.S. Pat. No. 4,578,114 discloses a thermal spray composite powder comprising an alloy constituent of nickel, iron or cobalt with aluminum and/or chromium, and elemental constituents aluminum and yttrium oxide. As background, it is indicated therein (column 3, lines 10-17) that chromium as an alloying element in a powder core coated with aluminum improves corrosion resistance, but reduces bond strength. According to the patent yttrium oxide is added to improve the bond strength. A similar powder is disclosed in U.S. Pat. No. 4,578,115 wherein cobalt is the additional component to improve bond strength. Thermal sprayed coatings of both of these types of composite powders are recommended for high temperature applications including cylinder walls of combustion engines. A similar powder is also disclosed in U.S. Pat. No. 3,841,901 wherein molybdenum is an additional component to improve machinability of the coatings.
Iron based coatings in cylinder bores are generally known and desirable for their scuff resistance and lower cost, being especially useful for enhancing aluminum engine blocks. U.S. Pat. No. 3,991,240 discloses a composite powder of cast iron core clad with molybdenum and boron particles, coatings thereof being suggested for plasma spraying onto cylinders walls. U.S. Pat. No. 3,077,659 discloses an aluminum cylinder wall of an internal combustion engine flame sprayed with a mixture of powdered aluminum with 8% to 22% powdered iron. Canadian patent No. 649,027 discloses cast iron sleeves for diesel engines with sprayed layers of molybdenum bond coat, intermediate chromium, and carbon steel final coating. U.S. Pat. No. 3,819,384 suggests coatings containing ferro-molybdenum alloy for various applications including cylinder liners. The aforementioned U.S. Pat. No. 2,588,422 discloses aluminum cylinders with thermal sprayed steels on a molybdenum bond coat.
U.S. Pat. No. 5,080,056 discloses aluminum cylinder bores for automotive engines thermal spray coated with aluminum bronze. These coatings are effected with an arc wire process or a high velocity oxygen-fuel process.
It is well known in the art of thermal spraying that bonding of coatings on the inside bores of cylinders is more difficult than on flat or external cylindrical surfaces. This is because there are inherent shrink stresses in the coatings due to the quenching effects in spray particles on the surface. These stresses are particularly apparent on an inside diameter to cause a coating to pull away and lift off. Moreover, aluminum substrates typically provide lower bond strengths than iron. Therefore, unless a material is self-bonding, an aluminum cylinder bore generally must be grit blasted which, as pointed out above, is undesirable. Exceptions may be spray materials that are soft enough to relieve stresses, such as the bronze of the aforementioned U.S. Pat. No. 5,080,056.
In summary, iron based coatings are of particular interest for applications such as cylinder bores, especially for aluminum alloy engine blocks for decreasing vehicle weights and costs while increasing performance, mileage and longevity. Also of interest is an ability to apply the coatings to smooth machined surfaces in one step without grit blasting. However, iron based coatings apparently have not, so far, been known in practice to bond well to smooth surfaces, even with compositing of aluminum with the iron. This situation is exacerbated for inside aluminum cylinder walls such as in combustion engines.
Therefore, an object of the present invention is to apply iron base thermal spray coatings having improved bonding. Other objects are to effect such coatings by thermal spraying onto smooth surfaces. A further object is to provide an improved method for applying iron base coatings to cylinder bores. Another object is to provide an internal combustion engine block with improved cylinder bore coatings. Yet another object is to provide a novel iron base powder which is particularly useful for thermal spraying in cylinder bores of internal combustion engines.